The default encoding for SQL Server Management Studio is UTF-16. If you are saving a file with this encoding, some diff tools or viewers "think" the file is binary and won't give you a comparison.
You can prevent this by "Save with Encoding..." as shown below. Just click the down arrow on the save button.
Now you'll be able to see diffs in GitHub (for instance).
I hope this is helpful, but if you know another way, or better way, I'd love to know about it, thanks!